The need for sensing position, speed or acceleration is growing, particularly in the automotive field. Anti-lock braking systems, traction control systems, electric power steering, four-wheel steering and throttle control are examples of functions that can use such sensing.
For such applications, it is desirable to have a position sensor (speed and acceleration can be derived from a position signal) that is rugged and reliable, small and inexpensive, capable of low (including zero) speed sensing and relatively immune to electromagnetic field interference from the other systems used in an automobile.
A well known form of position sensor is a semiconductor magnetoresistive sensor. Such a sensor comprises a magnetic circuit that includes two basic parts. One of these parts, typically kept stationary, includes a semiconductive sensing element that is sensitive to the magnetic flux density passing through its surface, and further includes a permanent magnet for creating a reference flux. The other of the two parts, termed the exciter, includes a high magnetic permeability element with a series of teeth that moves with relation to the stationary element for changing the reluctance of the magnetic circuit and for causing the magnetic flux through the sensing element to vary in a fashion corresponding to the position of the teeth.
Such a sensor is sensitive to the magnetic flux density rather than to the rate of flux density change and so it does not have a lower speed limit. This also makes it less sensitive to E.M.I. Moreover, its response is predictably related to the distribution of flux density over the surface of the sensing element.
Typically, the stationary part includes a magnetoresistive element including a semiconductive element whose resistance varies with the magnetic flux density passing through it in controllable fashion so that an electrical output signal can be derived. Moreover, when this magnetoresistor is produced from a high electron mobility semiconductor, such as compound semiconductors like indium antimonide or indium arsenide, a large electrical output signal can be available. If the output signal is sufficiently large, there is the possibility of providing an output signal that requires little or no further amplification, a factor of considerable advantage.
It is desirable to have a position sensor of high sensitivity so that a large electrical output signal can be produced efficiently and of easy manufacture so that it can be made reliably and at low cost.
The magnitude of the flux variations in the sensing element for a given change in position of the exciter is an important factor in determining the sensitivity of the sensor. Accordingly, a variety of designs have been attempted hitherto to maximize the change in the flux density through the sensor in response to a given change in exciter position. Typically, these attempts involved including a flux guide for the permanent magnet included in the stationary part of the magnetic circuit to provide a return path for the magnetic field of the magnet. Additionally, sometimes a field concentrator of commensurate size has been provided contiguous to the magnetoresistive element to concentrate flux through the magnetoresistive element.
However, for example, such techniques have typically produced magnetic circuit sensitivities no higher than about five percent for a typical exciter design having a three millimeter tooth pitch and one millimeter gap, where the sensitivity is defined as the difference between the maximum and minimum flux densities sensed divided by the mean flux density sensed (half the sum of the maximum and minimum flux densities sensed).